1. Field of the Invention
This invention relates to a permanent magnet-excited rotor for a synchronous machine which has a short-circuit cage with shorting rings arranged at both end faces and the lamination stack of which, secured against centrifugal stresses, has preferably radial slots for receiving the permanent magnets.
German Pat. No. 24 12 307 shows a rotor having a lamination stack in which individual segments are held by positively engaging undercuts at the shaft. Permanent magnets inserted into the radial slots are magnetized from the outside, after the short-circuit cage has been cast.
German Utility Model No. 77 26 439 suggests closing each of the radial slots at the outer circumference by a narrow strip, which also holds the lamination stack segments together and thereby secures them against centrifugal stresses. Such a securing device is substantially simpler than fastening of the laminated stack segments to the rotor shaft by positive engagement.
If the radial slots are closed at the outer circumference of the rotor, the permanent magnets must be inserted axially into the slots. Inserting the permanent magnets from the shaft opening is impossible as a rule since the radial dimension of the permanent magnets is usually larger than the diameter of the shaft opening. Axial insertion of the permanent magnets, however, must take place before the short circuit cage and the short rings are made, since the slots are covered up by the shorting rings. Permanent magnets which are inserted before the short circuit cage is cast are demagnetized by the high temperature developed during the casting of the cage and must, therefore, be remagnetized from the outside. Such subsequent remagnetizing is not possible with rotors with six or more poles and generally not if rare earth magnets are used.